1. Field of the Invention
The present invention relates generally to interconnections for semiconductor devices, and more particularly, to semiconductor device multilayer interconnection layers having improved interfusion barrier characteristics. The invention relates further to manufacturing methods therefor.
2. Description of the Background Art
The following characteristics are required for an interconnection layer which electrically connects in a semiconductor device an active element such as a transistor and a diode to another active element or to a passive element such as a resistor and a capacitor on a chip,
(1) Interconnection bulk resistivity is small,
(2) Contact resistance to a material to be connected thereto is small and formation of an ohmic contact is possible,
(3) Formation of film is easy and formation of a miniaturized interconnecting pattern is possible,
(4) High immunity to electromigration and erosion, and high reliability.
At present no material is found which fills all these requirements. However, an aluminum interconnection has the smallest interconnection resistance and is easy to be manufactured, so that it is most widely used at present. However, as a semiconductor device has been highly integrated in recent years, interconnection structure is miniaturized. As a result, various problems occur. These problems will be described in reference to FIGS. 5A and 5B. FIGS. 5A and 5B are partially sectional views of a structure of a semiconductor device including an aluminum interconnection layer. In the drawing, an n type impurity region 2 is formed on a surface of a p type silicon substrate 1. A major surface of the p type silicon substrate 1 is covered with an interlayer insulating film 3. A contact hole 4 is formed in the interlayer insulating film 3. The contact hole 4 reaches a surface of the n type impurity region 2. An aluminum interconnection layer 5 is formed in the interlayer insulating film 3 and the contact hole 4. The aluminum interconnection layer 5 is connected to the surface of the n type impurity region 2 at a bottom of the contact hole 4.
In the structure shown in FIG. 5A, interdiffusion occurs between the aluminum interconnection layer 5 and the p type silicon substrate 1 when heat treatment is applied during manufacturing. As a result, the aluminum breaks into the p type silicon substrate 1 to form a large number of concaves. This is generally referred to as alloy spike phenomenon. Then, an alloy spike 6 tends to break a pn junction which is formed in a shallower manner as a degree of integration is increased, resulting in leakage or short-circuit.
In order to prevent the alloy spike phenomenon, the structure shown in FIG. 5B is used in which the aluminum interconnection layer 5 includes silicon. However, in this case, a silicon nodule 7 is formed in the contact region between the surface of the p type silicon substrate 1 and the aluminum interconnection 5, thereby increasing a contact resistivity, which poses a new problem.
As another solution to the alloy spike phenomenon or a formation of a silicon nodule, an interconnection structure using a barrier metal layer has been proposed. The structure will be described in reference to FIGS. 6A and 6B. First as shown in FIG. 6A, a titanium layer 8 of a film thickness of about 1,000 .ANG. is formed on a surface of an interlayer insulating film 3 and in a contact hole 4 using sputtering method.
Then, as shown in FIG. 6B, the titanium layer 8 is annealed in a nitriding atmosphere such as nitrogen (N.sub.2) or ammonium (NH.sub.3) gas. A titanium nitride (TiN) layer 9 is formed on a surface of the titanium layer 8 by this thermal process. In addition, a titanium silicide layer 10 is formed in a contact portion between the titanium layer 8 and the surface of the p type silicon substrate 1. Film thickness of the titanium layer 8, the titanium nitride layer 9 and the titanium silicide layer 10 are 700-800 .ANG., 200-300 .ANG. and 2000 .ANG., respectively. Usually for thermal processing, silicidation is several times faster in reaction rate than nitriding. Therefore, the titanium silicide layer 10 is formed having a very large film thickness as compared with the titanium nitride layer 9. Thereafter, the aluminum conductive layer 5 is formed on a surface of the titanium nitride layer 9. A protection film 11 of a plasma CVD oxide film is formed on a surface of the aluminum conductive layer 5.
In the structure shown in FIG. 6B, the barrier metal layer has a double-layered structure formed of the titanium silicide layer 10 and the titanium nitride layer 9. The titanium silicide layer 10 forms a good ohmic contact with the n type impurity region 2 formed on the p type silicon substrate 1. In addition, the titanium nitride layer 9 acts as a barrier for preventing interdiffusion between the aluminum conductive layer 5 and the p type silicon substrate 1.
In the barrier metal layer of the double-layered structure, problem is that a film thickness of the titanium silicide layer 10 is thicker than that of the titanium nitride layer 9. More specifically, in the above described annealing step, since reaction of the titanium silicide layer 10 is fast, silicidation in the n type impurity region 2 proceeds before the formation of the titanium nitride layer 9 of a predetermined film thickness, which might destroy a shallow junction of the n type impurity region 2 and the silicon substrate 1.
A further improvement is introduced to overcome this problem. A second example of the interconnection structure including a barrier metal layer will be described in reference to FIGS. 7A and 7B.
First as shown in FIG. 7A, a titanium layer 8 of a film thickness of about 200 .ANG. is formed on a surface of an interlayer insulating film 3 and in a contact hole 4. Furthermore, a titanium nitride layer 9 of a film thickness of about 1,000 .ANG. is deposited on a surface of the titanium layer 8 using reactive sputtering method.
Then, as shown in FIG. 7B, by a thermal process in the nitriding atmosphere, a titanium silicide layer 10 is formed in a contact region between the titanium layer 8 and the n type impurity region 2. The titanium silicide layer 10 is formed to have a film thickness of about 300-400 .ANG.. Thereafter, an aluminum conductive layer 5 is formed. In addition, a protection film 11 such as plasma CVD oxide film is formed on a surface of the aluminum conductive layer 5.
The above described structure is described, for example, in "The Use of Titanium-Based Contact Barrier Layers in Silicon Technology" by C. Y. TING, et al. Thin Solid Films 96 (1982) 327-345.
In such a structure, the titanium nitride layer 9 having a barrier characteristic can be formed to have any film thickness. In addition, a film thickness of the titanium silicide layer 10 formed in the contact portion with the interconnection layer can be made small. However, the titanium nitride layer 9 is formed by reactive sputtering method. The titanium nitride layer 9 formed by reactive sputtering has a crystal structure which is not a close-packed structure and has a poor barrier characteristic. FIG. 8 is a schematic diagram of the crystal structure of the titanium nitride layer 9 formed by the reactive sputtering. Referring to the drawing, the titanium nitride layer 9 has a crystal structure in which crystalline grains extend in the form of columns. In such a crystal structure, there occurs grain boundary diffusion that silicon or the like diffuses along the grain boundary. Accordingly, it has the disadvantage that interdiffusion between the interconnection layer 5 and the n type impurity region 2 could not be completely prevented.